Electrodes, anodes and cathodes of electrochemical cells, and in particular polymer type cells (e.g., such as described in U.S. Pat. No. 5,296,318) having carbon anodes and spinel cathodes such as of LiMn.sub.2 O.sub.4 and electrolyte separators made of porous cast fluoropolymers, require a certain minimum of porosity to permit for ionic and reaction material flow.
Porosity is normally readily achieved in electrode structures, comprised of compacted particles a s a result of the inherent spacing between the particles. However, in polymer type electrochemical cells, the electrodes and electrolyte separators are made by casting, extrusion or other similar type methods, wherein a non-porous sheet of material is formed by the process.
In order to achieve sufficient porosity in such cast or extruded electrodes or separators, the materials comprising the electrodes or separators further include a homogeneously dispersed inert extractable material, such as the commonly utilized dibutyl phthalate (DBP) plasticizer in an amount (usually related to the volume of the electrode or separator) which will provide micro-porous passageways of the desired porosity in the electrodes or separators when the filler material is extracted. The filler material is inert relative to the electrode or separator material with which it is admixed.
In a common method utilized in the prior art, the filler material is removed by selective solvent extraction wherein the filler material is selected to be fully soluble in an extractant solvent and the electrode or separator material is insoluble in the is extractant solvent.
Typical solvents used in standard techniques are volatile organic solvents such as diethyl ether, petroleum ether, methanol, ethanol and the like. These substances are however flammable, expensive, explosive and toxic and are on the EPA's list of controlled chemicals. In addition, such materials are difficult to use and dispose of properly.
It is therefore an object of the present invention to provide a means for production of porous electrodes and electrolyte separators for electrochemical cells, and particularly polymer cells, by removal of filler materials therefrom with an inexpensive, relatively safe, filler extractant material.
It is a further object of the present invention to utilize said filler extractant material with substantially complete and rapid filler removal, without degradation or removal of materials used in the electrodes and separators of said cells.